The interrogating laser of a lidar system is used to cause emissions that are detected by a receiver of the lidar system. The detected emissions can be used to determine spatial distributions and other parameters and materials. The emissions may result from any number of processes (e.g., Mie scattering, Raleigh scattering, nonlinear scattering, scatter from hard targets, parametric processes, Raman scattering, fluorescence, laser induced breakdown etc.).
Lidar systems are either monostatic or bistatic. Monostatic optical arrangements share the same aperture for transmitting and receiving. These arrangements have the advantage of optical returns being focused onto relatively fast and small detectors regardless of the range of the interrogated material. Moreover, a single scan mirror can be used for directing the interrogating laser beam and the receiver's field of view. This common scanner for transmitting and receiving allows the system to have a relatively small field of view that results in relatively low background signatures. However, when emissions are produced near the receiver, they generally provide much larger detected signals than emissions generated at more distant ranges, thus making detection of materials farther away more difficult.
Bistatic optical arrangements use separate apertures for the laser and receiver. In these arrangements, the focusing optics for the receiver are generally configured so that near range emissions do not come to focus at the receiver. The field of view of the receiver should be large enough to accommodate imaged illumination patterns from the laser beam path. However, an enlarged field of view generally results in more background radiation being collected, which reduces the signal to noise ratio.